Impulse-reaction propulsion cycle for mole

ABSTRACT

This disclosure describes a propulsion cycle for a subterranean burrowing device. This cycle is characterized by the impact of an oscillating hammer upon an interior anvil, and by the return of the hammer to its starting position by force of a reaction piston acting on the hammer. While the reaction piston does work on the hammer, the opposite force thus produced on the mole nose causes a further soil penetration. The hammer eventually comes to rest and is returned to the anvil by a unidirectional constant bias force. The cycle is inherently self-timed.

PATENTEDFE'B 15 |972 SHEET 1 AUF 3 h Qmv i l Mw.

JC. COVNE /Nl/ENTORS AR SM/TH ATTORNEY vUnited States Patent Coyne etal.

[451 Feb. l5, 1972 [54] IMPULSE-REACTION PROPULSION CYCLE FOR MOLE [72]Inventors: `James Christopher Coyne, New Providence; Arnold Ray Smith,Chester,

both of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: July 17, 1970 [21] Appl. No.; 55,818

[52] U.S. Cl ..173/91, 173/119, 173/135 [51] ..B25d 9/00 [58] FieldofSearch ..173/119,120,134,135,91;

[56] References Cited UNITED STATES PATENTS 2,748,750 6/1956 Altschuler..173/119 3,465,834 9/1969 Southworth, Jr. ..173/91 3,137,483 6/1964Zinkiewicz ....173/91 3,407,884 10/1968 Zygmunt et al 173/91 PrimaryExaminer-J ames A. Leppink Attorney-R. J. Guenther and Edwin B. Cave[571 ABSTRACT This disclosure describes a propulsion cycle for asubterranean burrowing device. This cycle is characterized by the impactof an'oscillating hammer upon an interior anvil, and by the return ofthe hammer to its starting position by force of a reaction piston actingon the hammer. While the reaction piston does work on the hammer, theopposite force thus produced on the mole nose causes a further soilpenetration. The hammer eventually comes to rest and is returned to theanvil by a unidirectional constant bias force. The cycle is inherentlyself-timed.

6 Claims, S Drawing Figures HYDRAULIC LlNES T0 mwmmm .1am ...E .lr11/111111111111111111111111111/1//11111111111111111/1/111111/1/1111/1111111111111111111/11/11l/1M -1111111111 a y( WORKING SURFACE AREA BPATENTEBFEB 15 :an

SHEET 2 0F 3 umm o 1 lMPULSE-REACTION PROPULSION CYCLE FOR MOLE FIELD OFTHE INVENTION This invention relates to subsoil penetrators, or moles";and more specifically to a mole propulsion system.

BACKGROUND OF THE INVENTION The mole is a guided subsoil missiledesigned to form tunnels for the placement of utility services such astelephone distribution cable or service wire.

A typical mole propulsion system is described, for example, in H.Southworth, Jr. U.S. Pat. No. 3,465,834, issued Sept. 9, 1969. Withinthe moles cylindrical shell, a hammer is mounted to shuttle back andforth, impacting against an anvil at the missile's nose for forwardmotion, and at a back anvil for the backing mode. The hammer ishydraulically powered through hoses dragged along behind and connectingto surface service equipment. Each impact moves the mole through soil asmall amount depending on hydraulic pressure, pressurized area of thehammer, hammer mass, mole body mass, soil properties, impact velocity,and the mechanical properties of hammer, anvil and mole shell.

The propulsion cycle of this typical example begins with the applicationof hydraulic pressure to the hammer when situated at the rear end of itstravel. The mole shell reacts against the tunnel walls with a forcewhich is equal and opposite to the hydraulic force on the hammer andwhich must be less than the backward slip resistance of the tunnel. Thisforce accelerates the hammer over its full stroke. At impact, energy andmomentum of the hammer are transferred to the mole body causing the moleto advance. The hammer is returned to its initial position by switchingthe hydraulic pressure from behind the hammer to infront of the hammerat the instant of impact. When the hammer was traveled at least half wayback to its initial position, the hydraulic pressure is switched againto the rear end of the hammer.

In the above-described propulsion design, conversion .efficiency ofhydraulic energy at the pump to impact energy is typically about 30percent. The conversion efficiency of impact energy to soil penetrationis also typically about 30 percent. The overall efficiency thus is lessthan l percent. The consequences of this include high oil temperatures,severe shock and vibrations, reduced penetration speed, and an increasedprobability of stalling in rocky soil.

More specifically, for practical purposes the fraction of kinetic energyin the impacting hammer which is converted into soil penetration inaverage and weak strength soils is about equal to the hammer massdivided by the mole body mass. The remaining energy is lost in shellvibrations which, in addition to wasting energy, creates fatigue andshock problems.

Accordingly, one object of the invention is to increase the penetrationefficiency of a mole.

Another object of the invention is to reduce the shell vibration,particularly the fatigue and shock aspects.

A further object of the invention is to simplify the hydraulicsassociated with timing and hammer drive-in armole.

ln hard rocky soils the penetration efficiency decreases until the pointis reached where all the impact energy goes into elastically straining.the mole and the soil. The force on the soil never reaches the soilsyield force and no penetration occurs. Under these hard rocky soilconditions, it is desirable that the hammer have a large impact velocityto reduce the possibility of stalling.

Accordingly a further object ofthe invention is to match the impactvelocity of the hammer to the soil hardness, that is, to automaticallyprovidea high impact velocity (full stroke) for hard soil conditions anda slower impact velocity (partial stroke) for average and weak soilconditions.

SUMMARY OF THE INVENTION y In the present invention, a constantunidirectional bias force is applied to the hammer, tending toaccelerate it toward the front anvil. On strikingthe anvil,'the hammertriggers a sudden release of stored energy, which is directed to forcingthe hammer and anvil apart. Thus, the `hammer is propelled backwardagainst the constant bias force; but the moles nose is given an addedforward impetus into the soil.

In a particular inventive embodiment, the force is developedhydraulically through the release from an accumulator of hydraulic fluidunder substantial pressure. The fluid acts upon a reaction pistonmounted in axial relation to the hammer. The pressurized area of thereaction piston is substantially greater than the pressurized area ofthe hammer. The piston thus propels the hammer back against the biasforce. This reaction kick augments the forward impulse occasioned by thehammer-anvil impact. l

The above systems potential for higher penetration efficiency arisesbecause of its higher total energy transfer per cycle. That is, the workdone on the soil per cycle is the sum of the energy transfer by theimpacting hammer and the work done on the mole body (propelling throughsoil) by the hydraulics during the reaction piston stroke.

Also of substantial advantage Ais the fact that the control of thehammer cycle.y is inherently adaptive to differing soil conditions. Thehydraulic work done by the reaction piston is automatically proportionedbetween returning the hammer and propelling the mole into soil such thathammer impact velocity on the front anvil increases with increasing soilstrength.

A further advantage of this cycle is that the rcontrol of the hammercycle (i.e., control of hammer stroke) is virtually unaffected by systempressure. Thus, pump pressure can be changed without any substantialchange in hammer stroke. This is because both the hydraulic force on thereaction piston and the bias force on the hammer are derived from thesame pressure source.

A still further inventive advantage is that the mole can be reversedmerely by switching the hydraulic supply and return at the surfacecontrol station.

DESCRIPTION OF THE DRAWING FIG. l is a schematic sketch of a typicalmole operating environment;

FIG. 2 is a sectional side view of the impulse reaction drive of theinvention;

FIGS. 3 and 4 are schematic diagrams of the drive reversing mechanism;and

FIG. `5 is a graph depicting the inventive cycles performance undervarious soil conditions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT*C FIG. l depicts a molel0 in operation in the soil. Typically, the mole consists of a body lland anarticulated tail section 12 joined to guidance and poweringequipment on the surface throughout the umbilical cord 13 that includeshydraulic hoses and power connections.

Within the mole l0, a hammer 20 is centrally mounted upon a ported shaft21 for `fore yancl aft movement within a cylinder 22. Hammer 20 shuttlesbetween a rear position at which its back surface 20a is coincident withthe plane I9 seen edge-on in FIG. 2; and a forward position where itsfront surface 20b impacts upon the anvil surface 26 or mole body. Plane19 is slightly forward of a rear anvil 19a, and represents the plane atwhich, for hardest soil conditions, the hammer must come to rest forforward penetration of the mole.

An interior chamber 23 is bounded at one end by rings 17 fixed to shaft2l, and at the far end by a forward piston surface 18 having a workingsurface area denoted A. High pressure hydraulic fluid from supply linel5 enters chamber 23 via port 16. The high .pressure in chamber 23 actsagainst the piston surface I8, biasing the hammer 20 in the direction ofarrow 14. Of course, the length of chamber 23 changes as `the hammermoves.

Reaction piston 27 is also centrally mounted upon shaft 2l and ismovable within the chamber 28 substantially between the position shownin FIG. 2 and the chamber end 28a. The working surface area of piston27, denoted B is greater than the working or pressurized area A ofhammer 20. In one typical embodiment B/A=l0. The ratio B/A may vary fromto 20, however, within the teaching of the invention.

Forward of piston 27 a poppet valve 35 is mounted on shaft 2l. Afour-way spool valve 29 is ported as shown schematically in FIG. 2 wherethe valve is in its unoperated position. In this mode, valve 29 connectshigh-pressure hydraulic fluid from supply line l5, via the reservoir 32for poppet valve 35, and via duct 34, to the chamber 33 of reactionpiston 27. At this time, valve 29 alsoconnects the return-line chamber30 with reaction piston chamber 28 via passage 31. Under theseconditions` the supply line pressure in reservoir 32 maintains poppetvalve 35 snugly in its seat as shown in FIG. 2. A spring 35a biasingpoppet valve 35 establishes the extent of opening of that valve.

Propelled by the unidirectional hydraulic bias force in chamber 23, thehammer moves forward. As it is about to strike the anvil 26, itdepresses a plunger 36, causing increased hydraulic pressure in chamber29a which operates spool valve 29. Chamber 28 is thus switched fromreturn pressure to high pressure and the pressure in chamber 28 andreservoir 32 is equalized. By virtue of the greater area of the nose ofpoppet valve 35 upon which the high pressure is acting, however, poppetvalve 35 is cracked open. When this occurs, high-pressure fluid rushesfrom the reservoir 32 into chamber 28, thus driv-` ing reaction piston27 rearward against hammer 20 which is thereby accelerated rapidly awayfrom the anvil 26.

To accomplish this motion, it is necessary to provide an accumulator 44for a source of hydraulic energy close to the point of use. Accumulator44 is contiguous with supply liney 15 and reservoir 32. Although shownonly schematically in FIG. 2, accumulator 44 in practice is anair-pressurized bladder contained in an annulus.

The reaction to the accelerating force on hammer 20 acts on the missilenose 39, forcing the latter further into the soil for the duration ofthe time that reaction piston 27 is accelerating the hammer 20. Thisforce, in conjunction with the impulse of the hammer-anvil impact,propels the missile forward.

ln its operated position, subsequent to hammer-anvil impact, spool valve29 is latched by high-pressure hydraulic fluid applied through duct 34and latching passage 31a, both of which are of small diameter and lowflow capacity. When reaction piston 27 has traveled the prescribeddistance, it uncovers a low-pressure port 40 to the return line 4l. Theresulting sudden pressure drop in chamber 28 first reverses the force onpoppet valve 35, causing it to close. Secondly, the pressure dropunlatches spool valve 29 by dropping the pressure in chamber 29a throughlatching passage 31a. Valve 29 is returned by bias spring 43 to itsunoperated position shown in FIG. 2. This occasions areswitching ofpressures, the high pressure going once again to return chamber 33. Thisposition of valve 29 insures that chamber 28 remains at return pressurelevel such that poppet 35 remains closed. The pressure acting upon theface 42 returns the reaction piston 27 to its initial position.

The return stroke of reaction piston 27 takes place while the hammer 20coasts away from the anvil against its unidirectional bias force presentin chamber 23. The kinetic energy imparted to hammer 20 during itsacceleration by reaction piston 27 enables the hammer to coast againstthe biasing force until coming to rest at a point not beyond plane 19.Thereafter, this biasing force propels the hammer 20 back toward theanvil to start the next cycle.

REVERSING As seen in FIG. 2, the rear end of the mole body containshydraulic apparatus substantially identical to that found in the forwardend which has just been described. The components are identified byprimed numerals, like numerals corresponding to the components earlierdescribed.

In the rear end hydraulics, a reaction piston 27' is used having muchless diameter and working face area than piston 27, since in the reversemode, less work need be done on the hammer 20. Similarly, theaccumulator 44 has a much lower capacity.

Reversal capability requires the valve denoted 50 in the forward end,and 50 in the rear end connected to the hydraulic positions shown as A,B, C, D and A', B', C', D in FIG. 2. Valve 50 is depicted in detail inFIGS. 3 and4 as consisting of a chamber 5l and a plunger 52 with arcuatepassages 53, 54. The function of valve 50 is conventional` i.e., to givean output at outlet C of the higher of two pressures present at A and B,regardless of which of the latter positions experiences the higherpressure.

In the embodiment shown in FIG. 2, it is possible to reverse directionby switching the pressure on supply and return lines l5, 4l so that high(supply) pressure occurs in line 4l and low (return) pressure occurs inline l5. This is advantageously done at the surface station. Thereversing valves 50, 50' in response operate lto provide the now-highpressure of line 4l to the ducts 34 and 34', just as was present inthe'se passages in the forward penetration mode. Similarly, low pressureis provided to the passages 3l, 3l. With high pressure in the line 4l,hammer 20 moves to the left under its bias force, striking the plunger36' and then the rear anvil 19a. Plunger 36', operated, actuates thehydraulics at the mole rear end, in the manner already described for thefront end, causing the piston 27 to work against hammer 20 acceleratingit to the right. The resultant reaction kicks the mole rearward. Hammer20 in this mode is accelerated rearward no farther than plane l9b, sothat plunger 36 is not tripped. While this takes place, the front endhydraulics are locked in the mode depicted in FIG. 2 for reversing,which avoids its oscillating. Similarly to the same end, the rear endhydraulics are locked in the mode depicted in FIG. 2 when the mole is inits forward penetration mode.

Analysis shows that a substantial improvement in penetration efficiencyis obtained with the foregoing propulsion cycle `over prior propulsioncycles which return the hammer at the same force level as thataccelerating the hammer` to impact. The penetration efficiency is heredefined as the ratio of work done penetrating soil per cycle to thekinetic energy of the impacting hammer. The analysis shows that thegreater the force used in returning the hammer, the greater theimprovement in` the penetration efficiency; also the shorter therequired stroke of the reaction piston to produce a desired hammerstroke. A

straightforward energy balance, neglecting pressure drops,

gives the following useful relationship.

Return Force Bias Force Thus the advantage of a largearea return pistonis evident. Such a large area return piston demands a large hydraulicflow over a brief time, necessitating the presence ofthe accumulator 44as well as a fast-opening large flow passage poppet valve 35. Theaccumulator undergoes rapid discharge once each cycle, supplying theenergy for the hammer return immediately after each impact, andthereafter is recharged from the supply line during the remainingposition of each cycleA The graphs shown in FIG. 5 help demonstrate someof the beneficial properties of this propulsion cycle. The followingthree assumptions are made in the analysis leading to these graphicalresults. l The soil presents a purely Coulomb-type resistance topenetration. That is, the soil possesses negligible inertia andcompliance. This is a reasonable and valid assumption for the presentsituation where the soil is deformed far beyond its elastic range eachcycle and the anticipated cycle rate is slow, approximately l0 Hz. (2)The impacting hammer experiences insignificant rebound from the anvil.This is an experimentally observed fact, which is a consequence of themassiveness of the hammer relative to the anvil and nose structure ofthe mole. The major portion of the mole body mass is distributed along arelatively long shell. lt can be readily shown from the theorem ofconservation of linear momentum that the fraction of kinetic energy inthe impacting hammer transferred to the mole body in a nonvibrating modeis equal to the ratio of hammer mass to mole body mass. This means thatin a soil having Coulomb resistance, the penetration efficiency due tothe impacting hammer is also equal to this mass ratio. Finally, (3Pressure drops are neglected. This assumption does not alter thequalitative nature of the analytic results.

Four graphs are shown in FIG. 5 plotted against soil strength Fa (theCoulomb resistance of the soil) normalized by the biasing force Pa onthe hammer. The results presented in the figure were computed for aratio of hammer mass to mole body mass of one-third and a ratio ofreaction pistonarea to hammer area of l0. Sm, is the maximum hammerstroke, and l is the constant stroke of the return piston.

At high soil strength the penetration of the mole into soil is small asshown by curve 3. Thus the fraction of the output work of the reactionpiston going into soil penetration is small. This fraction is just /I,In this illustrative case the reaction piston stroke (l) is l0 percentof the maximum hammer stroke. Since nearly all the output work of thereaction piston goes into imparting kinetic energy to the returninghammer, the hammer stroke shown by curve l is nearly equal to themaximum hammer stroke. Also, for the reason that a small fraction of theoutput work of the reaction piston goes into soil penetration, thepenetration efficiency shown by curve 2 is only slightly greater thanthat shown in curve 4 for pure impact, which was stated to bev just themass ratio-in this case, one-third.

As the soil strength becomes less strong, the soil penetration (8)increases and therefore the fraction of the work output (/l) of thereaction piston going into soil penetration increases. Consequently theenergy imparted to the returning hammer becomes smaller and the hammerstroke is reduced as shown on curve l. The increased work output of thereaction piston going into soil penetration causes an increase inpenetration efficiency as shown on curve 2. Thus in average strengthsoils, the structure of the mole is not stressed by high velocityimpacts more than necessary and still the penetration efficiency isincreased. [n still weaker soils, those in which the Coulomb resistanceis less than five times the hammer bias force, the penetration per cycleinto soil greatly increases as shown on curve 3 and becomes anappreciable part of the hammer stroke. Hence, the distance that thehammer has to travel to return to its initial position is appreciablyreduced. Although a very small fraction of the work output of thereaction piston goes into imparting kinetic energy to the hammer, thissmall amount of energy is nevertheless sufficient to provide a furtherincrease in stroke as shown on curve l. Since nearly all the work outputof the reaction piston goes into soil penetration, the efficiencyfurther increases. At such weak soils, the cycle operation is betterdescribed as that of statically pushing through soil by virtue of thereaction force ofthe reaction piston, rather than that of impactpenetration.

lt is to be understood that the embodiments described herein are merelyillustrative of the principles of the invention. Various modificationsmay be made thereto by persons skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

l. A linear impacting unit for a subsoil penetrator, comprising:

a forward anvil fixed with respect to the penetrator nose,

said anvil defining the forward end of a cavity; a hammer mounted insaid cavity for impacting with said anvil; means for maintaining acontinuous forward bias on said hammer' a source of hydraulic energy;and

means responsive to hammer impact for applying said energy to saidhammer against said bias for a defined distance of hammer return travel.

2. Propulsion apparatus for a subsoil mole, comprising:

an elongated axial cavity in said device bounded by forward and rearanvils and containing an axially shuttling hammer;

means for biasing said hammer continuously toward said forward anvilwith a substantially constant first force; and

means for accelerating said hammer away from said first anvil on contacttherewith with a second force substantially greater than said firstforce, the energy imparted to said hammer by said second force beingcontrolled to limit hammer rearward travel to a point short of saidsecond anvil.

3. Apparatus pursuant to claim 2, wherein said biasing means and saidaccelerating means are both hydraulic and operated from the samepressure source.

4. Propulsion apparatus for a missilelike subsoil burrowing device,comprising:

an axial cavity within said device bounded by end walls comprisingforward and rear anvils;

a hammer mounted for fore-and-aft movement between said anvils andcomprising an interior chamber defined by a fixed wall and a forwardworking surface of area A;

separate hydraulic supply-and-return lines connected between said deviceand a remote hydraulic source;

means connecting said supply line and said chamber to bias said hammercontinuously toward said forward anvil;

a reaction piston having a working surface of area B where B A, mountedin a second chamber for fore-and-aft movement of said working surface Bfrom said forward anvil into said cavity;

a hydraulic accumulator connected to said supply line and to said secondchamber;

means for discharging said accumulator into said second chamber afterimpact of said hammer on said forward anvil, said reaction pistonaccelerating said hammer rearwardly, and thus said device forwardly, thework thereby done on said hammer being controlled to bring said hammerto rest before striking said rear anvil.

S. Apparatus pursuant to claim 4, further comprising a third interiorchamber defined between said fixed wall and a rear working surfacewithin said hammer;

a second reaction piston mounted in a fourth chamber for fore-and-aftmovement from said rear anvil into said cavity;

means for feeding hydraulic supply pressure into said third chamberthereby to bias said hammer into impacting contact with said rear anvilrather than said forward anvil; and

means operative after said impacting contact for accelerating saidsecond reaction piston against said hammer thus to drive the latterforwardly, and said device rearwardly.

6. Apparatus pursuant to claim 4 where the ratio B/A is in the range 5to 20.

1. A linear impacting unit for a subsoil penetrator, comprising: aforward anvil fixed with respect to the penetrator nose, said anvildefining the forward end of a cavity; a hammer mounted in said cavityfor impacting with said anvil; means for maintaining a continuousforward bias on said hammer; a source of hydraulic energy; and meansresponsive to hammer impact for applying said energy to said hammeragainst said bias for a defined distance of hammer return travel. 2.Propulsion apparatus for a subsoil mole, comprising: an elongated axialcavity in said device bounded by forward and rear anvils and containingan axially shuttling hammer; means for biasing said hammer continuouslytoward said forward anvil with a substantially constant first force; andmeans for accelerating said hammer away from said first anvil on contacttherewith with a second force substantially greater than said firstforce, the energy imparted to said hammer by said second force beingcontrolled to limit hammer rearward travel to a point short of saidsecond anvil.
 3. Apparatus pursuant to claim 2, wherein said biasingmeans and said accelerating means are both hydraulic and operated fromthe same pressure source.
 4. Propulsion apparatus for a missilelikesubsoil burrowing device, comprising: an axial cavity within said devicebounded by end walls comprising forward and rear anvils; a hammermounted for fore-and-aft movement between said anvils and comprising aninterior chamber defined by a fixed wall and a forward working surfaceof area A; separate hydraulic supply-and-return lines connected betweensaid device and a remote hydraulic source; means connecting said supplyline and said chamber to bias said hammer continuously toward saidforward anvil; a reaction piston having a working surface of area Bwhere B>A, mounted in a second chamber for fore-and-aft movement of saidworking surface B from said forward anvil into said cavity; a hydraulicaccumulator connected to said supply line and to said second chamber;means for discharging said accumulator into said second chamber afterimpact of said hammer on said forward anvil, said reaction pistonaccelerating said hammer rearwardly, and thus said device forwardly, thework thereby done on said hammer being controlled to bring said hammerto rest before striking said rear anvil.
 5. Apparatus pursuant to claim4, further comprising a third interior chamber defined between saidfixed wall and a rear working surface within said hammer; a secondreaction piston mounted in a fourth chamber for fore-and-aft movementfrom said rear anvil into said cavity; means for feeding hydraulicsupply pressure into said third chamber thereby to bias said hammer intoimpacting contact with said rear anvil rather than said forward anvil;and means operatIve after said impacting contact for accelerating saidsecond reaction piston against said hammer thus to drive the latterforwardly, and said device rearwardly.
 6. Apparatus pursuant to claim 4where the ratio B/A is in the range 5 to 20.